<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <link rel="stylesheet" href="../bootstrap/bootstrap.css" />
    <link href="https://cdn.bootcdn.net/ajax/libs/minireset.css/0.0.2/minireset.min.css" rel="stylesheet" />
  </head>
  <body>
    <style>
      div[class^='col'] {
        background-color: red;
      }
    </style>
    <div class="container">
      <div class="row">
        <div class="col-xl-4 col-sm-6 col-12">a</div>
        <div class="col-xl-4 col-sm-6 col-12">b</div>
        <div class="col-xl-4 col-sm-12 col-12">c</div>
      </div>
    </div>
  </body>
</html>
